1.I would not suggest making your own colors using gel pastes.Airbrushes are designed to use specific formulated liquids that are made for the airbrush only.If you try and water down gel or pastes the small particles will clog your airbrush and needle.You can mix airbrush liquids to get different hues.

2.There is really no good sites out there..You just have to get one and practice,practice,practice.

3.Roland Winbeckler did put out a airbrushing book several years ago but it is quite basic.I learned more by actually taking the airbrush and experimenting with papertowel and practice cakes.It's all about the control with gun and how light or heavy you go.